How is this adaptation? The branches of evergreen trees have developed a droop so that snow falls off them?

Answer 1

They do droop for this purpose.
But also consider their over all tree shape. They are conical with their drooping, flexible branches. Their shapes slope down and out so the snow can slide down the slope being shed from dipping branch to branch. The needles are also adapted to the cold with waxy coatings to conserve moisture. The ground is frozen so they can't replace the water once lost.

Answer 2

Heavy loads of snow can cause the branches to snap off, which is obviously harmful for the tree. Thus, developing a droop so that snow falls off will decrease harm done to the tree.